extern int _etext;
extern int _edata;
extern int _end;
These names refer neither to routines nor to locations with interesting contents; only their addresses are meaningful.
The address of _etext is the first location after the last read-only loadable segment.
The address of _edata is the first location after the last read-write loadable segement.
If the address of _edata is greater than the address of _etext, the address of _end is same as the address of _edata.
If the address of _etext is greater than the address of _edata, the address of _end is set to the page boundary after the address pointed to by _etext.
When execution begins, the program break (the first location beyond the data) coincides with _end, but the program break can be reset by the brk(2), malloc(3C), and the standard input/output library (see stdio(3C)), functions by the profile (-p) option of cc(1B), and so on. Thus, the current value of the program break should be determined by sbrk ((char *)0).
References to end, etext, and edata, without a preceding underscore will be aliased to the associated symbol that begins with the underscore.